The present invention relates to a method and apparatus for controlling the absorptance of an electrochromic layer.
Electrochromic layers become dark upon the application of a voltage or upon the feeding of charges, i.e., their absorptance increases (coloring). Upon the removal of charges, the layers again become transparent (decoloring).
It is known to utilize this phenomenon for display elements, in which case only the transparent and colored states are used. In this connection, to effect coloring, a defined current is supplied for a defined period of time. An excessive supply of charge must be avoided, if destruction of the electrochromic layer is to be avoided. On the other hand, the decoloring process is not critical since a greater charge cannot be removed from the layer than is present in it, and the completely charge-free layer is also the layer having the lower absorptance. One essential advantage of electrochromic layers is that they consume current only during the change in their absorptance.
For display elements, two states of the electrochromic layer are sufficient. However, for other purposes of use, as for sunglasses, dimmable mirrors and optical filters, intermediate values are advantageous or necessary. If identical charging and discharging times with identical charging and discharging currents, i.e., identical supply and removal of charges, are used for recycled charging and discharging between such intermediate values, then the layer will become brighter and brighter, since more charge is required for coloring of an electrochromic layer than can be removed for its decoloring.